Presently, servo control based on a sample servo system has been generally carried out in a magnetic disk device for the purpose of stably reading/writing data from/into each target track. The sample servo system is a system in which the track position of a recording/reading head is intermittently corrected based on a servo signal written in a servo frame in the head of each sector.
In the case where the sample servo system is used, the recording/reading head may be displaced from its original position due to fluctuation of the recording/reading head per se, deviation of track servo, etc. in a period from a point in time when the recording/reading head has passed through a servo frame to a point in time when the recording/reading head will pass through a next servo frame. In order to prevent the positional displacement of the recording/reading head between the servo frames from exerting influence on recording/reproducing characteristics, measures to widen the track pitch sufficiently has been hitherto taken.
Recently, demand for larger capacity of a magnetic disk device has increased, and reduction of the track pitch has been advanced in order to achieve higher density recording. Under these circumstances, the positional displacement of the recording/reading head in the sample servo system is becoming a critical problem. In order to solve this problem, it is considered that it is effective to replace the sample servo system with a servo control method using servo tracks.
The servo tracks are servo control tracks provided circumferentially in parallel with data tracks from/into which data are read/written. The track position of the recording/reading head is controlled while servo signals recorded in the servo tracks are read out. In this manner, the track position of the recording/reading head can be controlled not intermittently but consecutively to thereby effectively prevent occurrence of the positional displacement of the recording/reading head.
The servo control method using servo tracks has been disclosed, for example, in Patent Document 1 and Patent Document 2. In Patent Document 1 (Japanese Patent Laid-Open No. 91905/1998), there has been disclosed a system in which servo signals recorded in two servo tracks provided in inner and outer circumferential sides of a data track are read by one reading head to thereby perform servo control.
In Patent Document 2 (Japanese Patent Laid-Open No. 39986/1986), there has been disclosed a system in which a servo signal recorded in one servo track is read by two reading heads to thereby perform servo control, and a system in which servo information recorded in two adjacent servo tracks is read by one reading head to thereby perform servo control.
Recently, demand for higher transmission speed of a magnetic disk device as well as demand for higher capacity thereof has increased very strongly. In order to improve the transmission speed of the magnetic disk device greatly, it is a plurality of reading heads and a plurality of recording heads can be arranged in a direction of the radius of a magnetic recording medium so that data can be read/written in parallel from/into a plurality of tracks.
In the aforementioned servo control method disclosed in Patent Document 1, it is however impossible to improve the transmission speed by the provision of a plurality of reading heads because this method is based on the premise that a signal recorded in a data track and signals recorded in servo tracks in both sides of the data track are read by one reading head.
On the other hand, in the aforementioned servo control method disclosed in Patent Document 2, it is possible to provide a plurality of reading heads for reading signals from data tracks because each reading head for reading a signal from a servo track is independent of each reading head for reading a signal from a data track.
In the servo control method disclosed in Patent Document 2, the system for performing servo control by two reading heads has a problem in increase of the cost of the magnetic disk device because the system requires two reading heads for reading a servo signal in addition to reading heads for reading data signals. On the other hand, the system for performing servo control by one reading head has low efficiency because the system requires two servo tracks for every combination of data tracks from/into which data are read/written in parallel.
The invention was developed for solving the aforementioned problems of the background art. An object of the invention is to provide a magnetic disk device and a magnetic recording medium for inexpensively efficiently achieving stable servo control using servo tracks and increase of transmission speed due to parallel reading of data tracks.